The present invention relates to an optical fiber coupler.
In recent years a number of instruments have been developed which operate on measurements derived from coherent light which has been directed along a sensing path. Examples of such instruments are coherent communications devices, and also Mach-Zender, Sagnac or resonator interferometers for the sensing of rotation, magnetic fields, acoustic fields and other phenomena. The sensing path in such an instrument is provided by an optical fiber which is preferably of a lenght sufficient to provide adequate sensitivity, and which possesses optical properties which preserve the characteristics of the light being measured. The fibers may be polarization preserving.
Operation of the aforesaid instruments entails the dividing or mixing of optical signals along differently-directed paths, and thus requires a physical structure such as a fiber coupler which can couple a precise portion of the light from a segment of one fiber into a segment of another fiber. In applications sensitive to polarization, it is further required, that the coupler preserve the state of polarization of the optical energy which it couples from the first to the second fiber. This constraint may apply to devices having polarizing (PZ), polarization maintaining (PM) or single mode (SM) fibers.
In the prior art, one class of methods of coupling two fibers involves bonding each fiber in a shallow groove formed in a block, polishing each block to expose the fiber cladding close to the central fiber core, and then juxtaposing the two blocks, possibly also using an optical coupling medium, so that the evanescent wave energy from a finite segment of the exposed cladding of one fiber is coupled into a segment of the exposed cladding of the next fiber along an interval called the interaction length. The ratio of coupled energy depends upon the proximity to the core of the exposed cladding, the interaction length, and other factors of the construction.
FIGS. 1A and 1B illustrate such prior art constructions. In the embodiment shown in FIG. 1A, a fiber 10, with its protective jacket 13 removed from a region thereof, is embedded in an arcuate groove 12 formed in a block 14 so that it rises tangent to the surface 16 at the center of the block. The block surface 16 is curved so the fiber defines an extreme position, or the block is chosen to be softer than the fiber so that after polishing the fiber protrudes from the block surface.
As shown in FIG. 1A, two such blocks 14a, 14b are opposed to each other with their fibers placed in close proximity. Any gap between the fibers is filled by an optical index-matching medium 18 which may, for example, be an oil or an ultraviolet-curable adhesive of suitable refractive index. Such a structure allows the evanescent field regions of the two fibers to overlap, and coupling to occur with low loss. This construction has a disadvantage that light may be absorbed or scattered by the coupling medium 18, and, further, that temperature variations and aging effects can change both the dimensions and index of the medium 18, resulting in changes of the coupling ratio and increased loss. When the coupling medium is at all absorptive, another drawback of such an evanescent were coupler is an inherent asymmetric mode loss. For a resonant ring instrument, such as a Mach-Zender interferometer, asymmetric mode loss results in an asymmetric resonance dip, seriously complicating the instrument's performance.
Another prior art coupler construction is shown in FIG. 1B. This construction is similar to that of FIG. 1A, but differs in that the protruding cladding of each fiber is polished sufficiently flat so that the two opposing fibers may be placed in direct optical contact along their interaction length. For this construction, as described in U.S. Pat. No. 4,688,882 of Michael Failes, an adhesive 20 is used to secure and support the opposed blocks once their fibers have been placed in optical contact. The formation of a direct "optical contact" requires a much greater optical flatness, and a much lower level of surface roughness, than that required for the coupling of FIG. 1A, and also requires scrupulously clean fabrication conditions. However, the absence of an intermediate dissimilar medium in this construction results in improved stability of the coupling ratio and provides decreased loss.
Applicant believes, however, that this second construction is prone to several adverse eventualities. First, because the blocks themselves are primarily secured on opposing sides of a dissimilar medium, namely the adhesive 20, thermal gradients and thermal shock may be expected across the interface which may disrupt the fiber-to-fiber bond, especially at low temperatures. Secondly, the optical contact region may build up localized stresses of a type which can alter the refractive index and cause mode loss. The adhesive 20 provides a relatively low level of resistance to certain environment conditions (e.g., radiation) and may result in excessive outgassing. Finally, thermally induced stresses due to the differing materials of the fibers, the blocks and the adhesive, respectively, may compound some of the above effects.